docs: sync the Guide with the certificate pinning findings (Dart and Kotlin)

[dwikiriyadi]

Regenerated from the vault after srctool/sinew-dart#61 and srctool/sinew-kotlin#64.

Base Network

• Certificate pinning is part of the network package on both platforms: checked at the handshake, a failure reported, and ignored in the browser.
• Pin a backup.
• A pinning failure is never "offline", even behind another route's connection error.

Flutter Network

• pinCertificateAuthorities on the IO adapter, and a no-op on the web.
• Tested against real servers: the public-site check, and Apple's certificate requirements.

Kotlin Network

• SinewConfig.certificatePins applied on OkHttp and Darwin; an engine that can't pin throws.
• The dual-stack bug, where a pinning failure suppressed behind a refused connection was classified as offline.
• The iOS test runner's TLS limits.
• Tests of a guarded failure return the call's status rather than throwing unreachable.

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